The present invention relates to charge deposition printheads suitable for use in image forming systems. More particularly, the invention relates to printheads having selectively controlled electrodes, generally arranged at two or more levels in a laminated construction, that define a matrix array of charge-generating sites from which charge carriers are directed at an imaging member.
In an image forming system employing a charge emitting printhead, a charge latent image is comprised of charge carriers deposited on the surface of an imaging member. The imaging member moves along a process direction past a printhead, which produces a stream of charge carriers, such as electrons, from an array of charge-generating sites. The electrons are accelerated towards the imaging member in image configuration to create the latent charge image on the imaging member. The charge latent image receives a developer material, to develop the image, and the image is subsequently transferred and fused to a support sheet, such as paper, to form a printed document.
A charge emitting printhead generally includes a layer of long drive electrodes (e.g. RF-line electrodes), oriented in a first direction perpendicular to the process direction, and spanning a page width, and a layer of control electrodes (e.g. finger electrodes) oriented transversely to the drive electrodes to form spatially separated crossing points or intersections with the drive electrodes. A dielectric layer couples to, and physically and electrically separates and insulates, the drive electrodes from the control electrodes. The crossing points form charge-generating sites, which generate and direct toward the imaging member a collection of charge carriers that comprise the latent image. The drive electrodes are activated with an RF signal of up to several thousand volts amplitude while lesser bias or control voltages are applied to the control electrodes to switch between an ON and OFF emission of one polarity particles from the particular sites. The activation of the drive electrodes and the control electrodes creates localized charge source regions located at or near the crossing points of the drive electrodes and the control electrodes (the charge-generating sites) and allows charge carriers to escape from the glow or discharge regions and be accelerated to the imaging member. These printheads may be configured to deposit either positive or negative charge, and the negative charge may consist partly or entirely of either ions or electrons. Charge deposited by each charge emitting locus forms a small dot-like latent charge image on the imaging member as it moves past. Each raster scan of the printhead electrodes thus fills a narrow rectangular image strip, with the totality of image strips forming an image page.
In image forming systems using this type of printhead, the RF-driven electrodes extend generally along the width of the printhead in a cross-scan direction (i.e. perpendicular to the direction in which the imaging member moves), spanning many of the control electrodes which cross them at an angle. In one commercial embodiment, by way of example, twenty parallel RF lines extend the width of a print page, and are crossed by 128 oblique finger electrodes. During the time when one RF line is activated by a burst of approximately 5 to 25 cycles of a one-half to fifty MHz drive signal with a peak-to-peak amplitude of several thousand volts, the finger electrodes which cross the RF line electrodes at the desired dot locations are selectively biased to project charge dots from the printhead onto the imaging member. Each finger electrode effectively drives up to twenty charge emitting sites arranged along its length and corresponding to the twenty adjacent RF drive electrodes. By sequential activation of drive lines, the crossing sites of the same finger electrode are energized at slightly different times as the imaging member passes the printhead. In this manner, the finger electrode may deposit dots closely adjacent to each other on a single print line.
In an image forming system using the described printhead, a charge latent image is created line by line as an imaging member scans past the printhead. Each line extends the width of a print sheet, and is comprised of charge dots deposited in a fixed time sequence utilizing all of the charge-generating sites. Each charge-generating site corresponds to a specific pixel position along a cross-scan line in the image and is configured to place a charge dot at that particular location when activated. To create each line of the charge latent image on the imaging member, the drive electrodes are successively activated with a regular and fixed order as the imaging member scans past each electrode in the printhead. Thus, each line of the latent image is formed with the same dot deposition pattern as all preceding and all subsequent lines in the image.
The type of printheads discussed above are generally operated at a relatively small gap of about one-quarter millimeter from the imaging member, and are biased with respect to the imaging member to maintain a relatively high electric field which transports the charged particles across this gap. Generally, the amount of charge or charged particles which must be deposited to form an effective imaging dot is generally so great as to result in a considerable build-up of charge at the dot locus on the charge-receiving surface of the imaging member, relative to the magnitude of the acceleration potential. Thus, as a latent dot charge is formed, a local electric field develops which tends to deflect later arriving charge carriers directed at or near that dot. This effect may result in xe2x80x9cbloomingxe2x80x9d or enlargement of individual dots, such as described in the aforesaid U.S. Pat. Nos. 5,278,588 and 5,886,723, the contents of which are herein incorporated by reference, and various approaches are taught therein for addressing the precision of dot placement and image control to overcome deleterious the effect of dot blooming on image resolution. Surface charging effect may also slightly deflect nearby dots. This effect occurs when electrodes are actuated to lay down a latent charge dot on the imaging member at a position closely adjacent to or between one or more charge dots which have already been deposited along a line or region. In this case, the already deposited charge deflects the incoming charge carriers so that the subsequent dot is shifted laterally. Since the RF lines are few in number and are actuated in a generally fixed successive sequence, a vertical banding effect, known as xe2x80x9cVenetian blindingxe2x80x9d occurs. As each line of the image is formed with the same sequence of dot deposition, irregularities and defects are repeated at an equivalent location on every line of the image, generating a ripple or line of misplaced dots that appear as a streak or an anomalously light or dark band periodically crossing the face of the print. This banding effect tends to highlight and magnify even small defects in the image.
Despite the apparently high degree of uniformity of existing charge emitting printheads, a number of macroscopically visible irregularities are produced in the images which they deposit. These irregularities are repeated and magnified in every line of the image, creating a banding effect.
The present invention provides a printhead wherein a matrix array of charge-generating sites is defined by the crossing points of a first set of electrodes, such as drive electrodes, and a second set of electrodes, such as finger electrodes. Electrodes of the first set are parallel to each other and extend across the region to be printed, while electrodes of the second set are also parallel to each other, but extend obliquely across the first electrodes in a plane parallel thereto to define the crossing points. The crossing points of the first and second electrodes are closely spaced lattice points at which charge carriers are generated for projection onto a latent imaging member such that charge dots are uniformly deposited. In the matrix array of charge-generating sites, the rows of the matrix array are defined by the first set of electrodes, and the columns are defined by the second set of electrodes.
The printhead of the present invention includes a set of redundant drive electrodes forming in the matrix array. According to one practice, the set of redundant electrodes is formed by providing additional, surplus rows of the first set of electrodes, and modifying or adding to the second set of electrodes such that each redundant electrode repeats the charge-deposition pattern of another electrode in the matrix array. The redundant electrodes are selectively activated in place of the corresponding primary electrodes. This allows variation of the charge deposition sequence from line to line when forming a latent image.
Using a variety of sequence orders for depositing charge visually suppresses vertical banding effects in the final output image. The level of reduction of vertical banding corresponds to the number of redundant rows in the matrix array. As a result, image quality is significantly improved.